The present invention relates to a charged particle beam apparatus for performing highly accurate inspection, measurement and working by using a charged particle beam.
Available as a charged particle beam apparatus are a scanning electron microscope (hereinafter referred to as SEM) and an electron microscope for length measurement, simply, a measuring electron microscope (hereinafter referred to as CD-SEM) representing one of apparatus for inspection of semiconductor specimens. In addition, as described in JP-A-11-51886, an inspection apparatus is available which uses image comparison to thoroughly inspect the overall region to find where a defect exists on a wafer. There are a variety of inspection target objects involved in the inspection apparatus, including not only defective shapes in patterns but also conduction/non-conduction defects and deposition of foreign matters.
Desirability of customers for need for an image of higher resolving power and minute or fine formation of a semiconductor pattern prevails, followed by advancement of high resolution powering of the aforementioned apparatus, and there results a tendency to shorten the distance between an objective lens and a specimen. In other words, for the sake of high resolution powering, the focal point of the objective lens becomes shorter to ensure that a charged particle beam can be reduced in aberration and an image of high resolving power can be acquired. In the SEM, however, a field of view is searched with an observed image of a wide region (low magnification image) and hence there arises a problem that an image at a magnification suitable for view field search cannot be obtained. In the inspection apparatus, the above situation also gives rise to a causative factor of prevention of improvements in inspection speed. As described in the aforementioned JP-A-11-51886, with the aim of improving the throughput, a method is employed according to which a specimen stage is continuously moved while scanning an electron beam to acquire an image but with more improvements in throughput desired, it is necessarily inevitable to adopt a method of inspecting a wide region at once. But because of the shortened focal point, a problem is encountered in which when a wide region is scanned/irradiated with a charged particle beam, blur and distortion are caused at the peripheral portion. Therefore, the region of an image suitable for inspection is limited to that devoid of blur and distortion and the high resolution powering causes a reduction in throughput to incur.
In case an image is formed by an information signal and a line width of a pattern is measured or in the case of a comparative inspection for detecting a defect by comparing pattern images, the quality of an obtained image has the great influence upon the reliability of inspection results. Since the quality of an image is deteriorated owing to aberration of an electron optical system and to degraded image resolution caused by blur, the sensitivity of inspection of defects and the performance of measurement are degraded in the case of an image having blur and distortion at the peripheral portion under scanning/irradiation of a wide region. JP-A-10-318950 describes a method of assuring a view field of a wide region by detecting a positional shift through correction of the distortion to form an image devoid of distortion.
Usually, adjustment of focus and astigmatism is made while watching an image formed by an information signal but mostly, watching is directed to only the central portion of a region scanned with a charged particle beam. Exaggeratedly illustrated in FIG. 2 is the behavior of a charged particle beam near the surface of a specimen. The charged particle beam, as designated at 36, scans the specimen surface while tracing an arcuate locus like a pendulum about the center lying at an intersection 44 where an optical axis 43 crosses orthogonally a specimen 13 and actually, as the charged particle beam proceeds by width r, the height changes by h as shown in FIG. 2. The change h in height results in a shift of focal position of the charged particle beam, leading to an image field curvature aberration on the specimen responsible for blur, as shown at dotted line. Further, as the charged particle beam deviates from the optical axis, an astigmatic aberration also takes place. JP-A-2001-68048 discloses a method for automatic adjustment of astigmatism. And also, in scanning a wide region, the magnetic field and electric field of a deflector becomes irregular towards the peripheral portion and distortion takes place. The shorter the focus of the objective lens, the more the distortion becomes noticeable, indicating that as the high resolution powering advances, the limit of the scanning region tends to be narrowed. Conventionally, the scanning region is limited to a range within which any blur and distortion do not take place or they are permissible, thereby defining a maximum observation view field or a maximum inspection region.